Divalent organic cations as a novel protective layer for perovskite materials

Abstract

The traditional methylammonium lead iodide perovskite (MAPbI₃) is not stable under humid conditions, which limits its application. In this work, a new type of system is designed by replacing two MA cations of MAPbI₃ with a chain-like divalent organic cation lying on the surface. Density functional theory calculations are performed to study the geometric and electronic properties of this kind of novel system, such as the hydrogen bonding, energy bands, and optical properties. Three types of divalent cations have been tested, MA(CH₂)₃MA, MA(C₆H₄)MA, and MA(C₄H₂S)MA, denoted as ProDMA, PhDMA, and ThDMA, respectively. It is found that all three divalent cations can improve the thermal and moisture stability of perovskite materials. In particular, ProDMA, without a ring structure in the middle of the chain, has the most suitable size to fill into the available space on the surface where two MA cations are removed, and the corresponding system has the highest stability, especially for water-resistance. At the same time, the performance of the optical absorption and the power conversion efficiency are similar to that of the pristine MAPbI₃. This work suggests a promising strategy to improve the stability of perovskite materials by using divalent cations as the protective layer.